ESD hard backend structures in nanometer dimension

ABSTRACT

Some embodiments relate to a semiconductor device on a substrate. An interconnect structure is disposed over the substrate, and a first conductive pad is disposed over the interconnect structure. A second conductive pad is disposed over the interconnect structure and is spaced apart from the first conductive pad. A third conductive pad is disposed over the interconnect structure and is spaced apart from the first and second conductive pads. A fourth conductive pad is disposed over the interconnect structure and is spaced apart from the first, second, and third conductive pads. A first ESD protection element is electrically coupled between the first and second pads; and a second ESD protection element is electrically coupled between the third and fourth pads. A first device under test is electrically coupled between the first and third conductive pads; and a second device under test is electrically coupled between the second and fourth pads.

REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/255,739 filed on Nov. 16, 2015, the contents of which is herebyincorporated by reference in its entirety.

BACKGROUND

Integrated circuits (ICs), which often include millions or billions ofsemiconductor devices packaged within a single chip, are an underlyingtechnology for modern computers and mobile electronic devices. These ICsand their underlying semiconductor devices have in large part beenresponsible for ushering in the modern communications age.

Semiconductor devices of ICs can be damaged by electrostatic discharge(ESD) events. Such ESD events can occur when static electricity issuddenly discharged from a body surface to a device. For example, duringmanufacturing or testing of ICs, an ESD event can occur between anengineer's finger and a semiconductor wafer on which a semiconductordevice is located, causing a sudden in-rush of current or voltage tostrike the semiconductor device. This sudden in-rush of current orvoltage can catastrophically damage the device in a number of ways, suchas blowing out a gate oxide or causing junction damage, for example. ESDprotection circuits have been developed to protect against such ESDevents.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a perspective drawing of a chip that includes anumber of conductive pads in accordance with some embodiments.

FIG. 2 illustrates a cross-sectional view of the chip of FIG. 1 inaccordance with some embodiments.

FIG. 3A illustrates a schematic diagram of test pad configuration inaccordance with some embodiments.

FIG. 3B illustrates a schematic diagram of test pad configuration inaccordance with other embodiments.

FIG. 3C illustrates a schematic diagram of test pad configuration inaccordance with other embodiments.

FIG. 4 illustrates a cross-sectional view of a portion of a test padconfiguration in accordance with some embodiments.

FIG. 5 illustrates a top view of a device under test (DUT) of a test padconfiguration in accordance with some embodiments.

FIG. 6 illustrates some embodiments of a method of testing a chip inaccordance with a test pad configuration.

FIG. 7 illustrates a schematic diagram of another test pad configurationin accordance with some embodiments.

FIG. 8 illustrates a schematic diagram of another test pad configurationin accordance with some embodiments.

FIG. 9 illustrates an example layout of a fuse in accordance with someembodiments.

FIG. 10 illustrates an example layout of a fuse in accordance with someembodiments.

FIG. 11 illustrates an example layout of a fuse in accordance with someembodiments.

FIG. 12 illustrates a layout relating to a test pad configuration inaccordance with some embodiments.

FIG. 13 illustrates a layout view of a DUT consistent with FIG. 12 inaccordance with some embodiments.

FIG. 14 illustrates a layout view of a fuse consistent with FIG. 12 inaccordance with some embodiments.

FIG. 15 illustrates a perspective drawing of a chip that includes anumber of conductive pads in accordance with some embodiments.

FIG. 16 illustrates a cross-sectional view of a chip of FIG. 15 inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Integrated circuits (ICs) include one or more semiconductor devicesarranged in and/or on a semiconductor substrate. To implement desiredfunctionality in electronic devices (such as cell phones, computers,automotive control systems, and the like), ICs can be coupled to oneanother and/or can be coupled to external circuitry through conductivepads. As used herein, the term “conductive pad” can include a conductivebump, a conductive ball such as a solder ball, or some other conductivepad, pin, or bump with a rounded, planar, or substantially planarconductive surface, and/or can include a conductive landing padconfigured to be contacted by a wafer probe, network analyzer, etc.

FIG. 1 shows an example of a chip 100 having an arrangement ofconductive pads 102 which can connect an integrated circuit (IC)included in the chip 100 to external circuitry, such as another IC, aprobe tester, a breadboard, or a printed circuit board for example. FIG.2 shows a cross-sectional view of FIG. 1's chip 100 and its conductivepads 102. It will be appreciated that the chip 100 and its conductivepads 102 of FIGS. 1-2 merely illustrate a general example which does notlimit the present disclosure in any way. The present disclosure isapplicable to any type of chip at various stages of manufacture,including fully or partially fabricated chips included on asemiconductor wafer prior to dicing, 3DICs with multiple substrates thatare stacked over one another, chips which are in the process of beingdiced/packaged, and/or diced and fully packaged chips, such a packagedchip in a dual in-line package (DIP), flip-chip package, ball grid arraypackage, contactless package, through-hole package, and/or surface mountpackage, among others.

As shown in FIG. 2, chip 100 includes a semiconductor substrate 104 thatincludes one or more semiconductor devices 111 (such as transistors,diodes, etc., where details of the semiconductor devices 111 have beenomitted for simplicity/clarity). A passivation layer 106 and/or polymerlayer 108 are optionally present, depending on to what extent the chiphas been fabricated. One or more of the semiconductor devices and/orother features on the chip are configured to be electrically connectedto a corresponding conductive pad 102 through a conductive path. In FIG.2's example, the conductive path includes an interconnect structure 110,which is made up of a number of metal layers stacked over one anotherand which are connected to one another through conductive vias. In theillustrated example of FIG. 2, the interconnect structure includes ametal 1 line 112, a metal 2 line 114, and a metal 3 line 116, which areelectrically coupled by vias 118. It will be appreciated, however, thatany number of metal layers may be present depending on theimplementation. Although the conductive pads 102 can operably couple thesemiconductor devices 111 to one or more external circuits, theconductive pads 102 also make the semiconductor devices 111 and/or otherfeatures on the chip susceptible to ESD events from the externalenvironment.

ESD stress during manufacturing and packaging of chips is a seriousthreat, especially in technology nodes where IC features are on theorder of nanometers. For example, in current technology nodes, whereminimum feature sizes can be less than 20 nm or even less than 10 nm,one or more metal lines of interconnect structure 110 can have a widthcorresponding to the minimum feature size, and/or adjacent metal linescan be spaced so their pitch also corresponds to the minimum featuresize. An advantage of “shrinking” these metal lines 112-116, compared tometal lines of previous technology nodes, is that thinner, more closelyspaced metal lines allow the devices on the chip to be packed moredensely together. Unfortunately, the thinner, more closely spaced metallines are extremely fragile and vulnerable, and accordingly, are moresusceptible to damage due to ESD events. In particular, metal 1 line112, which is often thinner than higher metal lines on the IC (e.g.,thinner than metal 2 line 114, and thinner than metal 3 line 116), canbe damaged more easily than other metal lines and via structures.

The thinness of the metal 1 line 112 is particularly problematic whenelectromigration tests are to be carried out to characterize the metal 1lines 112 and/or other features of the chip 100. Electromigration is thetransport or “erosion” of material, such as metal atoms from metal 1line 112, caused by the gradual movement of the metal atoms due tomomentum transfer between conducting electrons and diffusing metalatoms. For modern ICs where metal 1 lines are very thin,electromigration tests are very difficult to conduct due to thefragility of the metal 1 lines. One approach to allow forelectromigration testing of a metal 1 line is to enlarge the length ofmetal 1 line to increase the parasitic resistance and reduce the failurerate. However, short metal lines having a length of less than 120micrometer cannot be monitored by such an approach, and a long line(such as 120 micrometers in length) does not accurately reflect typicalon-chip structures, since metal 1 lines are short in practicalinterconnect structures 110.

Therefore, the present disclosure provides techniques through which ametal 1 line 112, which is one example of a device under test (DUT), canbe tested while limiting risk of ESD damage and still reflecting typicalon-chip metal 1 line lengths used in production ICs. In someembodiments, the disclosure provides a test pad configuration in whichan arrangement of conductive pads, which make use of ESD protectiondevices and fuses, diverts at least a portion of the energy of any ESDpulse away from a short metal 1 line under test during manufacturing.Then, after manufacturing is complete and the manufacturer wants to runelectromigration tests, the manufacturer can blow the fuses and then runthe electromigration tests on the short metal 1 line. Because the fusesare blown, the ESD devices do not interfere with the electromigrationtest results. The concept is not limited to metal 1 lines as DUT, butcan also be used for other DUTs, including but not limited to: metal 2line 114, metal 3 line 116, higher or lower metal lines, as well asother device structures.

FIG. 3 shows a portion of a chip 300 exhibiting a test pad configurationaccording to some embodiments. The test pad configuration includes aseries of conductive pads 301, namely: first conductive pad 302, secondconductive pad 304, third conductive pad 306, and fourth conductive pad308. A first ESD protection element 310 is electrically coupled betweenthe first conductive pad 302 and the second conductive pad 304. A secondESD protection element 314 is electrically coupled between the thirdconductive pad 306 and the fourth conductive pad 308. A first deviceunder test (DUT) 318, which can manifest as a first short metal 1 linehaving a first resistance R1 in some embodiments, is electricallycoupled between the first and third conductive pads (302, 306), and asecond DUT 320, which can manifest as a second short metal 1 line havinga second resistance R2 in some embodiments, is electrically coupledbetween the second and fourth conductive pads (304, 308). The first ESDprotection element 310 has a third resistance, R3; and the second ESDprotection element has a fourth resistance, R4 . In some embodiments, R1and R2 are equal to one another, and are greater than R3 and R4. R1 andR2 can alternatively differ from one another, and R3 and R4 can be equalor un-equal different depending on the implementation. The firstconductive pad 302, the first DUT 318, the third conductive pad 306, thesecond ESD protection element 314, the fourth conductive pad 308, thesecond DUT 320, the second conductive pad 304, and the first ESDprotection element 310 are arranged as a closed current loop in someembodiments.

Referring briefly to FIG. 4, the conductive pads 301, for example thefirst conductive pad 302 and third conductive pad 306, are disposed overan interconnect structure 322 and over a semiconductor substrate 324,such as a bulk monocrystalline silicon substrate or asemiconductor-on-insulator (SOI) substrate. The interconnect structure322 can include a dielectric structure 326 such as silicon dioxide or alow-k dielectric material, and multiple metal layers arranged in thedielectric structure and which are connected by vias 321 extendingvertically between adjacent metal layers. A lowermost metal layer 328(e.g., a metal 1 layer) can include a metal 1 line 328L having a firstthickness, t₁, which is 20 nm or less for example; and an upper metallayer 330 (e.g., a metal 2 layer) can include metal2 lines 330L having asecond thickness, t₂, which is greater than the first thickness to helpreduce current crowding effects. Additional metal layers (not shown) canalso be arranged over the upper metal layer 330 and below the conductivepads 301. The conductive pads 301 often have a third thickness, t₃,which is greater than each of the first and second thicknesses t₁, t₂.The metal layers and conductive pads are often made of metal, such ascopper or a copper alloy, for example.

In some embodiments, the lowermost metal layer 328 can have a DUT region318R corresponding to the first DUT 318 which exhibits a thickness t₁ ofless than 20 nm, and a length of 120 micrometers or less from nearestedges of vias contacting an upper surface of the lowermost metal layer328. In some embodiments, the DUT region 318R can be electricallyisolated from semiconductor devices 311 on the semiconductor substrate324, which can allow accurate testing of the DUT region 318R to occurexclusively through the conductive pads 301. As shown in FIG. 5, in someembodiments the DUT region 318R can taper inwardly so the DUT region318R has a first width, w₁, of about 20 nm or less, while neighboringregions of the metal 1 line 328L are wider, for example by a factor oftwo or three or more, and have a second width, w₂, that is greater thanthe first width, w₁. In some embodiments, DUT region 318R corresponds toa metal 1 line having first and/or second widths w₁, w₂, ranging from 10nm to approximately 500 nm, and a length, L, of approximately 790 nm. Insome embodiments, the resistance (Rdut) of the first DUT 318 and/orsecond DUT 320 is approximately 360 ohm. The first and/or second ESDprotection element 310, 314 can be an Efuse (metal 1, W/L=20 nm˜0.5um/0.55 um) having a resistance (Rfuse) of approximately 165 ohm.

In general, the ESD protection elements 310, 314, can be bidirectionaldevices that allow current to flow through each of them in bothdirections. For example, the ESD protection elements 310, 314 can beimplemented as NPN or PNP bipolar junction transistors (BJTs), p-type orn-type MOSFETs, silicon controlled rectifiers (SCRs), Schottky diodes oravalanche diodes. Referring briefly to FIG. 3B's embodiment, one can seean example where the first ESD protection element 310 includes a firstfuse 312 and first and second diodes 313, 315; and where the second ESDprotection element 310 includes a second fuse 316 and third and fourthdiodes 317, 319. FIG. 3C illustrates another example where the first andsecond diodes 313, 315 are arranged in the opposite directions, as arethe third and fourth diodes 317, 319. It will be appreciated thatalthough FIGS. 3B and 3C depict first and second fuses 312, 316 as beingarranged in series between diodes, the first and second fuses 312, 316can be an inherent property of these bidirectional devices and are notnecessary a separately patterned structure.

FIG. 6 shows a methodology illustrating how the test pad configurationof FIG. 3 can be used as a part of a testing and/or manufacturingprocess for ICs. While the method described by the flowchart 600 isillustrated and described herein as a series of acts or events, it willbe appreciated that the illustrated ordering of such acts or events arenot to be interpreted in a limiting sense. For example, some acts mayoccur in different orders and/or concurrently with other acts or eventsapart from those illustrated and/or described herein. Further, not allillustrated acts may be required to implement one or more aspects orembodiments of the description herein, and one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

In 602, during manufacturing or testing, the chip 300 with its test padconfiguration is exposed to an electrostatic discharge (ESD) proneenvironment. For example, the chip 300 can be handled by an engineer orplaced on a wafer probe apparatus in which one or more wafer probes areplaced into electrical contact with one or more conductive pads 301.Because this environment may subject the chip 300 to ESD events, thefirst and second fuses 312, 316 are un-blown at this time, such that thefirst ESD protection element 310 provides a conductive path between thefirst conductive pad 302 and the third conductive pad 306, and thesecond ESD protection element 314 provides a conductive path between thesecond conductive pad 304 and the fourth conductive pad 308. If an ESDevent occurs, the first and second ESD protection elements 310, 314reliably dissipate the energy of the ESD event. For example, ifR1≈R3+R2+R4 and if the first and third conductive pads 302, 306 aregrounded and a positive ESD pulse event strikes the first conductive pad302, the ESD current can be split into two equal parts as it strikes thechip at 302 (and leaves the chip at 306). Thus, a first portion of theESD current can flow over R1, and a second portion of the ESD currentcan flow over first ESD protection element 310 and can continue over thesecond ESD protection element 314. In particular, the second portion ofthe ESD current can flow over first ESD protection element 310 whensecond diode 315 conducts in the forward direction and first diode 313is reversed biased and conducts through avalanche breakdown; and the ESDcurrent continues through the second ESD protection element 314 whenfourth diode 319 is reversed biased and conducts through avalanchebreakdown and third diode 317 conducts in the forward direction. Ratherthan the ESD current “spiking” over the first DUT 318 or the second DUT320, the current can be split, for example approximately equally underthis four pads scheme, which limits ESD stress on the first and secondDUTs 318, 320 and correspondingly limits ESD-induced damage and/orelectromigration.

The ESD current distributions can be as follows. For stress between thefirst conductive pad 302 and the second conductive pad 304:I _(R1) =I _(PAD1)*(R ₃)/(R ₁ +R ₂ +R ₃ +R ₄);I _(R3) =I _(PAD1)*(R ₁ +R ₂ +R ₄)/(R ₁ +R ₂ +R ₃ +R ₄)

For stress between the first conductive pad 302 and the third conductivepad 306:I _(R1) =I _(PAD1)*(R ₂ +R ₃ +R ₄)/(R ₁ +R ₂ +R ₃ +R ₄);I _(R3) =I _(PAD1)*(R ₁)/(R ₁ +R ₂ +R ₃ +R ₄)

For stress between the first conductive pad 302 and the fourthconductive pad 308:I _(R1)=½*I _(PAD1);I _(R3)=½*I _(PAD1)

It will be appreciated that in some embodiments R1 is approximatelyequal to R3+R2+R4 as set forth in the example just described. However,strict equality is not required, and it will be appreciated that R1 canalso differ from R3 +R2+R4. For example, R1 can be one or more orders ofmagnitude greater than R3 +R2+R4 in some embodiments. In otherembodiments, R1 can be greater than R3 +R2+R4 by 1%-500% in someembodiments, or by 5%-200% in other embodiments, or by 10%-100% in stillother embodiments. The more equal R1 is to R3 +R2+R4, the more equallyESD current will be split over the first DUT 318 and second DUT 320 inthe example above. However, even if current is not split equally, thediversion of some ESD current away from either first DUT 318 and/orsecond DUT 320 can help limit ESD-induced damage and/orelectromigration, and thus R1 can vary from R3 +R2+R4 (sometimessignificantly) and still provide improved functionality.

In 604, after manufacturing and/or testing, the first and second fuses312, 316 are blown, thereby breaking the current path where the firstand second fuses are located. The first fuse 312 can be blown by placingwafer probes into electrical contact with the first conductive pad 302and the second conductive pad 304. The wafer probes then apply anelectrical stress across the first and second conductive pads 302, 304,where the applied electrical stress is in the form of a current and/orvoltage of sufficient magnitude to blow the first fuse 312. For example,if we want to trim the first fuse 312, the stress can be applied on thefirst and second conductive pads 302, 304. The resistance in ESD fusepath will be R3 . In another path, the resistance is not R1 only, butR1+R4+R2. Thus, when R1 is approximately equal to R3 +R2+R4, theresistance in the ESD fuse path is R3 and the resistance in the otherpath (by simple substitution) is (R3 +R2+R4)+R4+R2, which is clearlygreater than R3 . This means most of current can be shunted to the firstfuse 312 under trimming, whereas the DUTs 320 and 318 can sustain therest of current (R_(DUT) _(_) _(path)>R_(fuse) _(_) _(path)).

Similarly, the second fuse 316 can be blown by placing wafer probes intoelectrical contact with the first and fourth conductive pads 306, 308,and applying electrical stress to the third and fourth conductive pads306, 308. The wafer probes then apply an electrical stress across thethird and fourth conductive pads 306, 308, where the applied electricalstress is in the form of a current and/or voltage of sufficientmagnitude to blow the second fuse 316. For example, if we want to trimthe second fuse 316, the stress can be applied on the third and fourthconductive pads 306, 308. The resistance in ESD fuse path will be R4. Inanother path, the resistance is not R2 only, but R1+R3 +R2. Thus, whenR1 is approximately equal to R3 +R2+R4, the resistance in the ESD fusepath is R4 and the resistance in the other path (by simple substitution)is (R3 +R2+R4)+R4+R2, which is clearly greater than R4. This means mostof current can be shunted to the second fuse 316 under trimming, whereasthe DUTs 320 and 318 can sustain the rest of current (R_(DUT) _(_)_(path)>R_(fuse) _(_) _(path)).

In 606, a determination is made as to whether electromigration testingis to be carried out on first DUT 318 and/or second DUT 320.

If so (i.e., “YES” at 606), electromigration testing is carried out onfirst DUT 318 and/or second DUT 320 in block 608. If not (i.e., “NO” at606), the method continues to 618 where additional testing, such as JTAGscanning, vector testing, and/or other chip verification and/orcharacterization is performed; and/or wafer dicing; and/or packaging canbe performed. Because electromigration testing can be carried out atvarious stages, note that these operations in block 618 are optional,and/or may be omitted and/or carried out prior to electromigrationtesting in block 608.

In 610, to conduct electromigration testing on first DUT 318, a firstseries of different current or voltage biases are applied across thefirst conductive pad 302 and third conductive pad 306. Thus, incrementalelectrical stresses are applied to first DUT 318.

In 612, after each current or voltage bias has been applied, first DUT318 is evaluated to determine to what extent metal from first DUT 318has been removed. For example, scanning electron microscopy (SEM)techniques can be used to inspect the chip after the current or voltagehas been applied, and the amount of electromigration can be determinedbased on changes in first DUT 318 before and after the testing. Forexample if first DUT 318 is a metal 1 line which has an initial width of20 nm and an initial length of 120 nm, a SEM measurement indicating thethickness of the metal 1 line has been reduced to 5 nm would indicate asignificant amount of electromigration has occurred. Rather than SEMmeasurements, resistance measurements of first DUT 318 can also becarried out to determine the extent of electromigration, if any. Forexample if a first DUT 318 metal 1 line had an initial resistance of 360ohm, and after testing the resistance increased to 1 kilo-ohm, asignificant amount of electromigration has likely occurred.

In 612, a second series of different current or voltage biases areapplied across second conductive pad 304 and fourth conductive pad 308to apply incremental electrical stresses to second DUT 320. This secondseries of different current or voltage biases can be the same as thefirst series of different current or voltage biases, or can be differentfrom the first series of different current or voltage biases.

In 614, after each current or voltage bias has been applied, evaluatesecond DUT 320 to determine to what extent metal from second DUT 320 hasbeen removed

FIG. 7 shows another test pad configuration 700 in accordance with someembodiments. The test pad configuration 700 includes a first conductivepad 702, second conductive pad 704, third conductive pad 706, and fourthconductive pad 708. A first DUT 710 is coupled between the first andsecond conductive pads, and a second DUT 712 is coupled between thethird and fourth conductive pads. A first fuse 714 is coupled betweenthe first conductive pad 702 and a shared node 716. A second fuse 718 iscoupled between the second conductive pad 704 and the shared node 716. Athird fuse 720 is coupled between the third conductive pad 706 and theshared node 716. A fourth fuse 722 is coupled between the fourthconductive pad 708 and the shared node 716.

FIG. 8 shows another alternative test pad configuration 800 inaccordance with some embodiments. The test pad configuration 800includes a first conductive pad 802, second conductive pad 804, thirdconductive pad 806, and fourth conductive pad 808. A first DUT 810 iscoupled between the first and second conductive pads, and a second DUT812 is coupled between the third and fourth conductive pads. A firstfuse 814 is coupled between the first conductive pad 802 and a sharednode 816. A second fuse 818 is coupled between the second conductive pad804 and the third conductive pad 806. A third fuse 820 is coupledbetween the fourth conductive pad 808 and the shared node 816.

FIGS. 9-12 show several examples of ways in which a fuse (e.g., firstfuse 312 and/or second fuse 316 in FIG. 3) can be implemented inaccordance with some embodiments. FIG. 9 shows a top view of a fuse 900in accordance with some embodiments. The fuse 900 is formed in a singlemetal layer (e.g., metal 1 layer), and includes a narrow strip 902arranged between first and second fuse terminals 904, 906, which arewide compared to the narrow strip 902. So long as the current betweenthe first and second fuse terminals 904, 906 is less than some maximumthreshold, the narrow strips 902 remains intact. To “blow” the fuse,current in excess of the maximum threshold is provided from one of thefuse terminals through the narrow strip 902 and out of the other fuseterminal. Because the narrow strip 902 has a smaller cross-sectionalarea through which current flows, current entering the narrow strip 902is “crowded” together and causes heating and stress that erodes thenarrow strip 902, thereby “blowing” the fuse 900 and breaking theelectrical connection between the first and second fuse terminals 904,906.

FIG. 10 shows a top view of another fuse 1000 in accordance with someembodiments. The fuse includes first and second metal 1 segments 1002,1004 extending in a first direction (e.g., x-direction), and a metal 2segment 1006 extending in a second direction. Vias 1008, 1010electrically couple the metal 2 segment 1006 to the first and secondmetal 1 segments 1002, 1004. So long as the current between the firstand second metal 1 segments is less than some maximum threshold, thevias 1008, 1010 remain intact. To “blow” the fuse, current in excess ofthe maximum threshold is provided from one of the metal 1 segmentsthrough the vias and metal 2 segment and out of the other metal 1segment. The vias 1008, 1010 are arranged to have a smallercross-sectional area than the metal 1 and metal 2 segments, such thatcurrent entering the vias is “crowded” together and causes heating andstress that erodes the vias, thereby “blowing” the fuse and breaking theelectrical connection between the first and second metal 1 segments.

FIG. 11 shows a top view of another fuse 1100 in accordance with someembodiments. The fuse includes multiple metal 1 segments 1102-1114extending in a first direction (e.g., x-direction), and a multiplemetal2 segments 1116-1126 extending in a second direction and beingcoupled to the metal 1 segments through vias. So long as the currentbetween an input fuse terminal (e.g., leftmost metal 1 segment 1102) andan output fuse terminal (e.g., rightmost metal 1 segment 1114) is lessthan some maximum threshold, the fuse remains intact. To “blow” thefuse, current in excess of the maximum threshold is provided into an endportion of the input fuse terminal (e.g., 1102), then passes through thevias and metal 2 segments before passing out of the output fuse terminal(e.g., 1114). The vias are arranged to have a smaller cross-sectionalarea than the metal 1 and metal 2 segments, such that current enteringthe vias is “crowded” together and causes heating and stress that erodesthe vias, thereby “blowing” the fuse and breaking the electricalconnection between input and output fuse terminals.

FIGS. 12-14 illustrate layout views of a test pad configuration inaccordance with some embodiments, with FIG. 12 illustrating an overalllayout view 1200 and FIG. 13 illustrating a more detailed view of a DUT1210 and FIG. 14 illustrating a more detailed view of a first fuse 1214.FIG. 12's layout illustrates a test pad configuration with a firstconductive pad 1202, second conductive pad 1204, third conductive pad1206, and fourth conductive pad 1208. A first DUT 1210, which is shownin more detail in FIG. 13, is coupled between the first conductive pad1202 and third conductive pad 1206, and a second DUT 1212 is coupledbetween the second conductive pad 1204 and fourth conductive pad 1208. Afirst ESD protection element, which includes a first fuse 1214 that isshown in more detail in FIG. 14, is coupled between the first conductivepad 1202 and second conductive pad 1204, and a second ESD protectionelement with a second fuse 1216 is coupled between the third conductivepad 1206 and the fourth conductive pad 1208. The first fuse 1214includes a narrow strip 1218 arranged between an input fuse terminal1220 and an output fuse terminal 1222.

The layout of FIGS. 12-14 can make use of “coloring” techniques toproduce features on the order of nanometers—which providesdensely-packed features, but which also leaves the features potentiallysusceptible to ESD damage. In “coloring” techniques, multiple masks areused for a single layer on the integrated circuit, such that theresultant single layer can have features that are more closely spacedthan can be achieved with a single exposure photolithography step. Forexample, in some embodiments the first DUT 1210 illustrated in FIG. 13can be made up of multiple horizontal conductive lines 1302-1326 whichare formed in a single metal layer, such as a metal 1 layer. Thehorizontal conductive lines can alternate between two different colorsin the vertical direction. Thus, a first conductive line 1302 can be ametal 1 line of a first color, a second conductive line 1304 can be ametal 1 line of a second color, a third conductive line 1306 can be ametal 1 line of the first color, a fourth conductive line 1308 can be ametal 1 line of the second color, and so on. Consequently, even if thelithography system used to form the conductive lines is capable ofachieving some predetermined minimum resolution for a single mask step(such as nearest edges of neighboring metal 1 lines which are formed bya single mask being separated by 20 nm); lines of different colors canhave edges that are separated by less than the predetermined minimumresolution (such as nearest edges of metal 1 lines of different colorsbeing separated by only 10 nm).

Vertical lines 1328-1334, which are also formed in metal 1 in thisexample, operably couple the narrow horizontal lines of the first DUT1210 to input and output DUT terminals (1336, 1338), which are widerthan the horizontal conductive lines 1302-1326 and vertical lines1328-1334. Thus, if electromigration tests are carried out on this firstDUT 1210, one or more predetermined current pulses can be provided tothe input DUT terminal 1336 through the first conductive pad 1202, forexample, and the current can flow over the horizontal and vertical linesand then exit through the output terminal 1338. The amount ofelectromigration occurring for the horizontal lines due to this currentcan vary depending on small variations in the manufacturing process.Thus, this DUT structure provides a reliable manner of allowingelectromigration to be evaluated, which has prior to this disclosure,been challenging, particularly at small feature sizes for technologynodes on the order of nanometers. In some embodiments, some of thehorizontal lines (e.g., 1302-1306) are left isolated or decoupled fromthe vertical lines 1328-1334, such that when the first DUT 1210 isevaluated by visual inspection, these decoupled lines 1302-1306 serve asa baseline of sorts by which any electromigration for the coupledhorizontal lines 1308-1326 can be compared.

FIG. 15-16 show an example of a chip 1500 having conductive pads inaccordance with some embodiments. It will be appreciated that the testpad configurations previously described in FIGS. 3-14 can make use ofthe conductive pads as shown in FIGS. 15-16 in some embodiments. As canbe seen in FIG. 15-16, a chip 1500 includes a substrate 1510 withintegrated circuits formed therein and/or thereon. The substrate 1510can manifest as a semiconductor substrate, including but not limited toa bulk silicon substrate, a semiconductor substrate, asilicon-on-insulator (SOI) substrate, or a silicon germanium substrate.Other semiconductor materials including group III, group IV, and group Velements are used for the substrate in some embodiments. The substrate1510, in some embodiments, further comprises a plurality of isolationfeatures (not shown), such as shallow trench isolation (STI) features orlocal oxidation of silicon (LOCOS) features. The isolation featuresdefine and isolate various microelectronic elements (not shown).Examples of such various microelectronic elements formed in thesubstrate 1510 in accordance with some embodiments include transistors(e.g., metal oxide semiconductor field effect transistors (MOSFET),complementary metal oxide semiconductor (CMOS) transistors, bipolarjunction transistors (BJT), high voltage transistors, high frequencytransistors, p-channel and/or n-channel field effect transistors(PFETs/NFETs), etc.); resistors; diodes; capacitors; inductors; fuses;and other suitable elements. Various processes are performed to form thevarious microelectronic elements including deposition, etching,implantation, photolithography, annealing, and other suitable processes.The microelectronic elements are interconnected to form the integratedcircuit device, such as a logic device, memory device (e.g., SRAM), RFdevice, input/output (I/O) device, system-on-chip (SoC) device,combinations thereof, and other suitable types of devices.

The substrate 1510 further includes, in some embodiments, inter-layerdielectric layers 1504 and a metallization structure made up of multiplemetal layers 1506 overlying the integrated circuits. The inter-layerdielectric layers 1504 in the metallization structure include low-kdielectric materials, un-doped silicate glass (USG), silicon dioxide,silicon nitride, silicon oxynitride, or other commonly used materials.The dielectric constants (k value) of the low-k dielectric materials is,in some embodiments, less than about 3.9, or less than about 2.8. Metallines 1506 in the metallization structure are, in some embodiments,formed of copper or copper alloys.

A contact pad 1512, which is one example of a conductive pad, is a topmetallization layer formed in a top-level inter-layer dielectric layer,which is a portion of conductive routes and has an exposed surfacetreated by a planarization process, such as chemical mechanicalpolishing (CMP), in some embodiments. Suitable materials for the contactpad 1512 include, but are not limited to, for example, copper (Cu),aluminum (Al), AlCu, copper alloy, or other conductive materials. In oneembodiment, the contact pad 1512 is made of metal (e.g., Al), which is,in some embodiments, used in the bonding process to connect theintegrated circuits in the respective chip to external features.

A passivation layer 1514 is formed on the substrate 1510 and patternedto expose a portion of the contact pad 1512 for allowing subsequent postpassivation interconnect processes. In one embodiment, the passivationlayer 1514 is formed of a non-organic material selected from un-dopedsilicate glass (USG), silicon nitride, silicon oxynitride, siliconoxide, and combinations thereof. In another embodiment, the passivationlayer 1514 is formed of a polymer layer, such as an epoxy, polyimide,benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, althoughother relatively soft, often organic, dielectric materials are alsousable.

A post passivation interconnect (PPI) process is then performed on thepassivation layer 1514. In some embodiments, an adhesion layer and/or aseed layer (1515) are formed on the passivation layer 1514. The adhesionlayer, also referred to as a glue layer, is blanket formed, covering thepassivation layer 1514. The adhesion layer includes commonly usedbarrier materials such as titanium, titanium nitride, tantalum, tantalumnitride, and combinations thereof, and is formed using physical vapordeposition, sputtering, or the like. The adhesion layer helps to improvethe adhesion of the subsequently formed conductive lines ontopassivation layer 1514. The seed layer is blanket formed on the adhesionlayer. The materials of the seed layer include aluminum, aluminum,alloys copper, copper alloys, silver, gold, aluminum, and combinationsthereof. In an embodiment, the seed layer is formed of sputtering. Inother embodiments, other commonly used methods such as physical vapordeposition or electroless plating are used.

A post passivation interconnect (PPI) line 1518 is formed on theadhesion layer and/or seed layer (1515, if used). Using a mask and aphotolithography process, a conductive material fills an opening of themask, followed by removing the mask and any exposed portions of theadhesion layer and seed layer. The removal includes a wet etchingprocess or a dry etching process. In one embodiment, the removalincludes an isotropic wet etching using an ammonia-based acid, which is,in some embodiments, a flash etching with a short duration.

The conductive material filling serves as the PPI line 1518. The PPIline 1518 includes, but is not limited to, for example, copper,aluminum, copper alloy, or other conductive materials. The PPI line 1518further includes, in some embodiments, a nickel-containing layer (notshown) on the top of a copper-containing layer. The PPI formationmethods include plating, electroless plating, sputtering, chemical vapordeposition methods, and the like. The PPI line 1518 connects the contactpad 1512 to bump features. The PPI line 1518 also functions, in someembodiments, as power lines, re-distribution lines (RDL), inductors,capacitors or any passive components. The PPI line 1518 in someembodiments has a thickness less than about 30 μm, for example, betweenabout 2 μm and about 25 μm.

A dielectric layer (not shown), also referred to as an isolation layeror a passivation layer, is formed in some embodiments on the exposedpassivation layer 1514 and the PPI line 1518. The dielectric layer isformed of dielectric materials such as silicon nitride, silicon carbide,silicon oxynitride or other applicable materials. The formation methodsinclude plasma enhance chemical vapor deposition (PECVD) or othercommonly used CVD methods.

A polymer layer 1522 is formed on the dielectric layer. Lithographytechnology and etching processes such as a dry etch and/or a wet etchprocess, are then performed to pattern the polymer layer 1522, thus anopening is formed to pass through the polymer layer 1522 and expose aportion of the PPI line 1518 for allowing subsequent bump process. Thepolymer layer 1522, as the name suggests, is formed of a polymer, suchas an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO),and the like, although other relatively soft, often organic, dielectricmaterials can also be used. In one embodiment, the polymer layer 1522 isa polyimide layer. In another embodiment, the polymer layer 1522 is apolybenzoxazole (PBO) layer. The polymer layer 1522 is soft, and hencehas the function of reducing inherent stresses on respective substrate.In addition, the polymer layer 1522 is easily formed to a thickness oftens of microns.

An under-bump-metallurgy (UBM) layer 1524 that includes, in someembodiments, a diffusion barrier layer and a seed layer are formed. TheUBM layer 1524 is formed on the polymer layer 1522 and the exposedportion of the PPI line 1518, and lines the sidewalls and bottom of theopening 1523. The diffusion barrier layer, also referred to as a gluelayer, is formed to cover the sidewalls and the bottom of the opening.The diffusion barrier layer is, in some embodiments, formed of tantalumnitride, titanium nitride, tantalum, titanium, or the like. Theformation methods include physical vapor deposition (PVD) or sputtering.The seed layer is, in some embodiments, a copper seed layer formed onthe diffusion barrier layer. The seed layer is, in some embodiments,formed of copper alloys that include silver, chromium, nickel, tin,gold, and combinations thereof. In one embodiment, the UBM layer 1524includes a diffusion barrier layer formed of Ti and a seed layer formedof Cu.

A mask layer (not shown) is provided on the UBM layer 1524 and patternedfor exposing a portion of the UBM layer 1524 for bump formation. Aconductive material with solder wettability, such as SnAg, or otherlead-free or lead-containing solder materials, is deposited on theexposed portion, thereby forming a bump (or ball) 1506, which can alsobe referred to as a conductive contact pad in some embodiments and whichis in contact with the underlying UBM layer 1524. The bump 1506 definesa connection ball for the semiconductor device 1500.

Thus, some embodiments of the present disclosure relate to asemiconductor device on a semiconductor substrate. An interconnectstructure is disposed over the semiconductor substrate, and a firstconductive pad is disposed over the interconnect structure. A secondconductive pad is disposed over the interconnect structure and is spacedapart from the first conductive pad. A third conductive pad is disposedover the interconnect structure and is spaced apart from the first andsecond conductive pads. A fourth conductive pad is disposed over theinterconnect structure and is spaced apart from the first, second, andthird conductive pads. A first ESD protection element is electricallycoupled between the first and second conductive pads; and a second ESDprotection element is electrically coupled between the third and fourthconductive pads. A first device under test is electrically coupledbetween the first and third conductive pads; and a second device undertest is electrically coupled between the second and fourth conductivepads.

In other embodiments, the present disclosure relates to a method oftesting a semiconductor device. In this method, the semiconductor deviceis subjected to an electrostatic discharge (ESD) prone environmentduring manufacturing or testing. After the semiconductor device has beenexposed to the ESD prone environment, the first fuse and second fuse areblown away. After blowing away the first and second fuses, anelectro-migration test is conducted by applying electrical stress to thefirst device under test or to the second device under test.

In still other embodiments, the present disclosure relates to.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of testing a semiconductor device, thesemiconductor device comprising: a semiconductor substrate; ainterconnect structure disposed over the semiconductor substrate; afirst conductive pad disposed over the interconnect structure; a secondconductive pad disposed over the interconnect structure and spaced apartfrom the first conductive pad; a third conductive pad disposed over theinterconnect structure and spaced apart from the first and secondconductive pads; a fourth conductive pad disposed over the interconnectstructure and spaced apart from the first, second, and third conductivepads; a first ESD protection element, including a first fuse,electrically coupled between the first and second conductive pads; asecond ESD protection element, including a second fuse, electricallycoupled between the third and fourth conductive pads; a first deviceunder test (DUT) electrically coupled between the first and thirdconductive pads; and a second DUT electrically coupled between thesecond and fourth conductive pads; the method comprising: subjecting thesemiconductor device to an electrostatic discharge (ESD) proneenvironment during manufacturing or testing of the semiconductor device;after subjecting the semiconductor device to the ESD prone environment,blowing away the first fuse and blowing away the second fuse; and afterblowing away the first and second fuses, conducting an electro-migrationtest by applying electrical stress to the first DUT or to the secondDUT.
 2. The method of claim 1: wherein the first DUT is a first metal 1line; and wherein the second DUT is a second metal 1 line.
 3. The methodof claim 2, wherein the first and second metal 1 lines each have a firstresistance, and the first and second ESD protection elements each have asecond resistance that is less than the first resistance.
 4. The methodof claim 1, wherein the first DUT comprises: a metal 1 line which iselectrically isolated from the semiconductor substrate; and first andsecond vias which are in direct contact with an upper surface region ofthe metal 1 line; wherein the metal 1 line has a first width in a DUTregion of the metal 1 line arranged between the first and second vias,and wherein the metal 1 line tapers outwardly to have a second widthunder the first and second vias, the second width being greater than thefirst width.
 5. The method of claim 1, wherein conducting theelectro-migration test by applying electrical stress comprises: applyinga first plurality of current or voltage biases across the first andthird conductive pads to electrically stress the first DUT; andevaluating the first DUT to determine to what extent metal from thefirst DUT has been removed by the electrical stress.
 6. The method ofclaim 5, wherein the first DUT corresponds to a first metal 1 line andthe second DUT corresponds to a second metal 1 line.
 7. The method ofclaim 6, wherein the first metal 1 line is electrically coupled to thefirst and third conductive pads through one or more vias in theinterconnect structure.
 8. The method of claim 5, wherein the first DUTcorresponds to a first metal 1 line and the second DUT corresponds to asecond metal 1 line, and further comprising: after the first and secondmetal 1 lines have been electrically stressed, evaluating the firstmetal 1 line and the second metal 1 line to determine to what extentmetal from the first and second metal 1 lines has been removed by theelectrical stress.
 9. A method of testing a semiconductor device, thesemiconductor device comprising: a semiconductor substrate; ainterconnect structure disposed over the semiconductor substrate; afirst conductive pad disposed over the interconnect structure; a secondconductive pad disposed over the interconnect structure and spaced apartfrom the first conductive pad; a third conductive pad disposed over theinterconnect structure and spaced apart from the first and secondconductive pads; a fourth conductive pad disposed over the interconnectstructure and spaced apart from the first, second, and third conductivepads; a first ESD protection element, including a first fuse,electrically coupled between the first and second conductive pads; asecond ESD protection element, including a second fuse, electricallycoupled between the third and fourth conductive pads; a first deviceunder test (DUT) electrically coupled between the first and thirdconductive pads; and a second DUT electrically coupled between thesecond and fourth conductive pads; the method comprising: subjecting thesemiconductor device to an electrostatic discharge (ESD) proneenvironment during manufacturing or testing of the semiconductor device;after subjecting the semiconductor device to the ESD prone environment,blowing away the first fuse and blowing away the second fuse; and afterblowing away the first and second fuses, conducting an electro-migrationtest by applying electrical stress to the first DUT or to the secondDUT; wherein the first conductive pad, the first DUT, the thirdconductive pad, the second ESD protection element, the fourth conductivepad, the second DUT, the second conductive pad, and the first ESDprotection element are arranged as a closed current loop.
 10. The methodof claim 9: wherein the first DUT is a first metal 1 line; and whereinthe second DUT is a second metal 1 line.
 11. The method of claim 10,wherein the first and second metal 1 lines each have a first resistance,and the first and second ESD protection elements each have a secondresistance that is less than the first resistance.
 12. The method ofclaim 10, wherein the first DUT corresponds to a first metal 1 line andthe second DUT corresponds to a second metal 1 line, and furthercomprising: after the first and second metal 1 lines have beenelectrically stressed, evaluating the first metal 1 line and the secondmetal 1 line to determine to what extent metal from the first and secondmetal 1 lines has been removed by the electrical stress.
 13. The methodof claim 9, wherein the first DUT comprises: a metal 1 line which iselectrically isolated from the semiconductor substrate; and first andsecond vias which are in direct contact with an upper surface region ofthe metal 1 line; wherein the metal 1 line has a first width in a DUTregion of the metal 1 line arranged between the first and second vias,and wherein the metal 1 line tapers outwardly to have a second widthunder the first and second vias, the second width being greater than thefirst width.
 14. The method of claim 9, wherein conducting theelectro-migration test by applying electrical stress comprises: applyinga first plurality of current or voltage biases across the first andthird conductive pads to electrically stress the first DUT; andevaluating the first DUT to determine to what extent metal from thefirst DUT has been removed by the electrical stress.
 15. The method ofclaim 14, wherein the first DUT corresponds to a first metal 1 line andthe second DUT corresponds to a second metal 1 line.
 16. The method ofclaim 15, wherein the first metal 1 line is electrically coupled to thefirst and third conductive pads through one or more vias in theinterconnect structure.
 17. A method of testing a semiconductor device,the semiconductor device comprising: a semiconductor substrate; ainterconnect structure disposed over the semiconductor substrate; afirst conductive pad disposed over the interconnect structure; a secondconductive pad disposed over the interconnect structure and spaced apartfrom the first conductive pad; a third conductive pad disposed over theinterconnect structure and spaced apart from the first and secondconductive pads; a fourth conductive pad disposed over the interconnectstructure and spaced apart from the first, second, and third conductivepads; a first ESD protection element, including a first fuse,electrically coupled between the first and second conductive pads; asecond ESD protection element, including a second fuse, electricallycoupled between the third and fourth conductive pads; a first deviceunder test (DUT) electrically coupled between the first and thirdconductive pads; and a second DUT electrically coupled between thesecond and fourth conductive pads; the method comprising: subjecting thesemiconductor device to an electrostatic discharge (ESD) proneenvironment during manufacturing or testing of the semiconductor device;after subjecting the semiconductor device to the ESD prone environment,blowing away the first fuse and blowing away the second fuse; and afterblowing away the first and second fuses, conducting an electro-migrationtest by applying electrical stress to the first DUT or to the secondDUT; wherein the first ESD protection element comprises: a first fusecoupled between the first conductive pad and the second conductive pad;a first diode in series with the first fuse, the first diode having acathode coupled to the first conductive pad and an anode coupled to thefirst fuse; and a second diode in series with the first fuse and firstdiode, the second diode having a cathode coupled to the secondconductive pad and an anode coupled to the first fuse.
 18. The method ofclaim 17: wherein the first DUT is a first metal 1 line; and wherein thesecond DUT is a second metal 1 line.
 19. The method of claim 18, whereinthe first and second metal 1 lines each have a first resistance, and thefirst and second ESD protection elements each have a second resistancethat is less than the first resistance.
 20. The method of claim 17,wherein the first DUT comprises: a metal 1 line which is electricallyisolated from the semiconductor substrate; and first and second viaswhich are in direct contact with an upper surface region of the metal 1line; wherein the metal 1 line has a first width in a DUT region of themetal 1 line arranged between the first and second vias, and wherein themetal 1 line tapers outwardly to have a second width under the first andsecond vias, the second width being greater than the first width.